Is Article 5 of Council Directive 2000/78/EC of 27 November 2000 establishing a general framework for equal treatment in employment and occupation to be interpreted as meaning that an employer has an obligation, in relation to a person who, due to his disability, is no longer capable of performing the essential functions of the post to which he was assigned, to assign him to another post, for which he has the requisite skills, capabilities and availability, where such a measure would not impose a disproportionate burden on the employer?’